simplify Readable in some ways

* make it single-param instead of multi-param
* get rid of its functional dependency

The cost is the use of another type family.  Seems like a win to me.

src/Tahoe/Download.hs

+{-# LANGUAGE TypeFamilies #-}
+
 {- | A high-level interface to downloading share data as bytes from storage
  servers.
 -}
@@ -41,7 +43,14 @@ type DownloadedShare = (ShareNum, LB.ByteString)
  given servers, if possible.
 -}
 download ::
-    (MonadIO m, Verifiable verifyCap, Readable readCap verifyCap) =>
+    -- To download, we require a capability for which there is a Readable
+    -- instance because are also going to decrypt the ciphertext.  A different
+    -- download interface that skips decryption could settle for a capability
+    -- with a Verifiable instance.  We also require that the Verifier type for
+    -- the read capability has a Verifiable instance because Verifiable is
+    -- what gives us the ability to locate the shares.  If we located
+    -- separately from decrypting this might be simpler.
+    (MonadIO m, Readable readCap, Verifiable v, Verifier readCap ~ v) =>
     -- | Information about the servers from which to consider downloading shares
     -- representing the application data.
     Map.Map StorageServerID StorageServerAnnouncement ->
@@ -150,7 +159,7 @@ locateShares servers lookupServer storageIndex required =
  decode them and decrypt the contents of possible.
 -}
 decodeShares ::
-    (MonadIO m, Readable readCap verifyCap) =>
+    (MonadIO m, Readable readCap, Verifiable v, v ~ Verifier readCap) =>
     -- | The read capability which allows the contents to be decrypted.
     readCap ->
     -- | The results of downloading the shares.

src/Tahoe/Download/Internal/Capability.hs

-{-# LANGUAGE FunctionalDependencies #-}
 {-# LANGUAGE TypeFamilies #-}
 
 module Tahoe.Download.Internal.Capability where
@@ -46,9 +45,15 @@ class Verifiable v where
         LB.ByteString ->
         Either (LB.ByteString, ByteOffset, String) (ShareT v)
 
-class (Verifiable v) => Readable r v | r -> v where
+{- | A capability which confers the ability to recover plaintext from
+ ciphertext.
+-}
+class Readable r where
+    -- | Represent the type of a Verifiable associated with the Readable.
+    type Verifier r
+
     -- | Attentuate the capability.
-    getVerifiable :: r -> v
+    getVerifiable :: r -> Verifier r
 
     -- | Interpret the required number of shares to recover the plaintext.
     --
@@ -57,7 +62,7 @@ class (Verifiable v) => Readable r v | r -> v where
     -- shareToCipherText :: r -> [(Int, ShareT r)] -> LB.ByteString
     --
     -- cipherTextToPlainText :: r -> LB.ByteString -> LB.ByteString
-    decodeShare :: MonadIO m => r -> [(Int, ShareT v)] -> m (Either DownloadError LB.ByteString)
+    decodeShare :: MonadIO m => r -> [(Int, ShareT (Verifier r))] -> m (Either DownloadError LB.ByteString)
 
 instance Verifiable CHK.Verifier where
     type ShareT CHK.Verifier = Tahoe.CHK.Share.Share
@@ -74,7 +79,9 @@ instance Verifiable CHK.Verifier where
  recover the original plaintext.  Additionally, it can be attentuated to a
  Verifiable.
 -}
-instance Readable CHK.Reader CHK.Verifier where
+instance Readable CHK.Reader where
+    type Verifier CHK.Reader = CHK.Verifier
+
     getVerifiable = CHK.verifier
     decodeShare r shareList = do
         cipherText <- liftIO $ Tahoe.CHK.decode r shareList
@@ -96,7 +103,8 @@ instance Verifiable SDMF.Verifier where
 
     deserializeShare _ = fmap (\(_, _, c) -> c) . decodeOrFail
 
-instance Readable SDMF.Reader SDMF.Verifier where
+instance Readable SDMF.Reader where
+    type Verifier SDMF.Reader = SDMF.Verifier
     getVerifiable = SDMF.readerVerifier
     decodeShare r shareList = do
         cipherText <- Right <$> liftIO (SDMF.decode r (first fromIntegral <$> shareList))